std: Add a Duration type

Taken from rust-chrono[1]. Needed for timers per #11189.
Experimental.

[1]: https://github.com/lifthrasiir/rust-chrono
std: Rename sleep, periodic, and oneshot timers to sleep_ms, etc.

Rename io::timer::sleep, Timer::sleep, Timer::oneshot,
Timer::periodic, to sleep_ms, oneshot_ms, periodic_ms. These functions
all take an integer and interpret it as milliseconds.

Replacement functions will be added that take Duration.

[breaking-change]
std: Add sleep, oneshot and periodic timers, taking Duration
std: Add comments to the time module